Silicon carbide has been known for more than 100 years and originally was formed by passing a strong electric current from a carbon electrode through a clay mixture. From the outset its abrasive qualities were judged outstanding, leading to the founding of The Carborundum Company in a development important to the abrasives industry. With time other qualities of silicon carbide were exploited, but its uses remained restricted largely to the metallurgical, abrasive, and refractory industries. More recently silicon carbide has been used as an important structural ceramic where its corrosion resistance and wear resistance may be advantageously employed.
Silicon carbide also is a semiconductor, having a conductivity intermediate to metals (conductors) and insulators or dielectrics, with a large band-gap at 300.degree. C. of about 2.2 eV for the cubic forms and about 2.86 eV for the non-cubic forms. Its excellent physical and electrical properties make it a candidate for the fabrication of electrical devices that can operate at significantly higher temperatures compared to devices based on silicon or on gallium arsenide. This wide band-gap semiconductor shows desirable properties such as chemical stability even at several hundred degrees centigrade, high thermal conductivity, high breakdown electric field, and high saturated electron drift velocity, all of which make silicon carbide a very attractive material for high temperature, high frequency, and high power electronic devices.
A brief word regarding terminology is in order before proceeding. Silicon carbide may crystallize in cubic, hexagonal, or rhombohedral structures. The cubic structure is referred to as .beta.-SiC, appears to exhibit only one polytype, and sometimes is designated as 3C-SiC. In contrast, there are a multitude of hexagonal and rhombohedral structures, where these non-cubic structures collectively are referred to as .alpha.-SiC. The hexagonal polytypes are designated as H-SiC and the rhombohedral-polytypes are designated as R-SiC. The most commonly studied polytype appears to be that designated as 6H-SiC. This is prepared by sublimation according to methods well known in the art and which will not be further discussed here. For the purpose of this application it is sufficient to note that various polytypes are known, are well described, and are not only readily made according to procedures taught in the prior art but also are readily available from several commercial sources. The various polytypes arise from long range order in the crystal but additional discussion here is unneeded since in practice our invention is independent of SiC polytypes.
Before silicon carbide can gain wide acceptance as a semiconductor it is necessary to develop methods of preparing SiO.sub.2 films at its surface and to prepare such films with a uniform, controlled thickness, without defects, and in a predictable fashion. Films of SiO.sub.2 act as the medium for the selective etching and film deposition essential in the fabrication of electronic devices, especially those of large scale integration (LSI) and very large scale integration (VLSI) design.
The thermal oxidation of silicon carbide to form films of SiO.sub.2 has been studied many times in recent years, especially at temperatures between about 1000.degree. and 1400.degree. C. For example, Meuhlhoff and coworkers studied the oxidation of clean surfaces of silicon carbide at both the carbon-rich face, SiC(0001), and the silicon-rich face, SIC(0001), and found that at 995.degree. C. oxidation was approximately equal at both surfaces. L. Meuhlhoff and coworkers, J. Appl. Phys., 60, 2558 (1986). They also confirmed that silica forms a protective layer for further oxidation which becomes controlled by diffusion of the reactant (oxygen) and reaction product (carbon monoxide) through the oxide layer.
Earlier E. Fitzer and R. Ebi ("Silicon Carbide 1973", J. W. Faust, Jr., and C. E. Ryan, editors, University of South Carolina Press, Columbia, 1974) had shown that at a low oxygen potential at the SiO.sub.2 /SIC interface the reaction of SiC with SiO.sub.2 leads to the formation of volatile SiO and CO. At low oxygen partial pressure in the primary stage SiO is formed at once which causes catastrophic oxidation. Meuhlhoff and coworkers also have shown that at 1300.degree. C. Si(g) vaporization from the carbon-rich face leads to extensive carbon enrichment and surface graphitization; J. Appl. Phys., 60, 2842 (1986).
Unfortunately the high temperature thermal oxidation of silicon carbide to form a silica film is fatally defective. It is extraordinarily difficult, if not impossible to obtain a uniform coat with good electrical qualities when the substrate is heated at a temperature greater than 600.degree.-700.degree. C. as required by the prior art. Certain oxidants, such as nitric acid, lead to occlusions in the coatings and other imperfections. It also is extremely difficult to obtain layers of well defined thickness using prior art methods.
In a search for a low-temperature method of oxidizing silicon carbide to form silica films, we have found that the product stream from microwave discharge induced decomposition of ozone is quite effective in oxidizing silicon in both silicon carbide and silicon films to form a silica film at temperatures under 200.degree. C. Using ozone as the feed gas for a microwave discharge plasma it is possible to produce localized, high concentrations of excited oxygen atoms near, e.g., a silicon carbide surface which through the subsequent reaction of the atomic oxygen with silicon leads to quite uniform coatings of silica on the SiC surface. Using the method which is our invention it is possible to produce silica layers of well defined thickness through control of various oxidation parameters. Because oxidation is done rapidly and at a comparatively low temperature, the semiconductor characteristics of the underlying silicon carbide remain unaltered. The silicon oxide films which are formed are relatively free of imperfections and their thickness can be readily controlled. Thus, the method which is our invention promises to bring silicon carbide from the realm of unfulfilled potential into the world of accomplished reality by making it possible to produce uniform films of SiO.sub.2 with relatively few imperfections in a manner which readily lends itself to controlling film thickness without adversely affecting the semiconductor properties of the underlying silicon carbide.
Although our initial labors concerned the formation of silica films on silicon carbide, that accomplishment, however significant it may be, is dwarfed by the more general observation that the product stream from microwave discharge induced decomposition of ozone is a powerful oxidant for a broad variety of otherwise oxidation-resistant materials at quite modest temperatures. Thus our invention in its broadest aspect is a novel method of oxidizing surfaces at relatively low temperatures, and is particularly important for the oxidation of surfaces which are readily oxidized only at relatively high temperatures of several hundred degrees centigrade. As we explain within, although our oxidations are performed at low temperatures, say, under 100.degree. C., oxidation proceeds as if the temperature were several thousands of degrees. This assumes special significance when the material, or rather the surface of the material, sought to be oxidized is itself thermally labile, whether structurally or chemically. The ramifications of our invention are many and will be better understood from the following exposition.